1. Prepare Your Equipment and Ingredients

1. Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
2. Ensure your eggs are at room temperature, as this helps create more volume when whipping the whites.

2. Melt the Chocolate and Butter

1. In a heatproof bowl set over a pot of simmering water (double boiler), melt the dark chocolate and butter together. Stir gently until smooth and glossy. Remove from heat and let it cool slightly.

3. Whip the Egg Yolks and Sugar

1. In a large mixing bowl, combine the egg yolks and granulated sugar. Beat with an electric mixer on medium speed until pale, thick, and creamy, about 3–4 minutes.
2. Stir in the melted chocolate mixture and vanilla extract, mixing until fully incorporated.

4. Whisk the Egg Whites

1. In a separate clean bowl, add the egg whites and a pinch of salt. Using a clean whisk or mixer, beat until stiff peaks form. This step is crucial for creating the light texture of the cake.

5. Fold the Whites into the Chocolate Mixture

1. Add one-third of the whipped egg whites to the chocolate mixture and fold gently to lighten it.
2. Add the remaining egg whites in two additions, folding carefully with a spatula to maintain as much air as possible.

6. Bake the Cake

1. Pour the batter into the prepared springform pan and smooth the top with a spatula.
2. Bake in the preheated oven for 35–40 minutes, or until the cake has risen and the top is slightly cracked. The center should still have a slight wobble.

7. Cool and Serve

1. Remove the cake from the oven and let it cool completely in the pan. The cake will deflate slightly as it cools, creating a dense, mousse-like texture.
2. Once cool, dust with cocoa powder or powdered sugar if desired. Serve with whipped cream and fresh berries for garnish.